1. Field of the Invention
The present invention relates to a power (capacity)-variable compressor and an air conditioner having the compressor, and particularly to a technique of performing power(capacity) control at multistage while reducing the power consumption.
2. Description of the Related Art
In most of recent air conditioners, the power control is performed at a heat source side (compressor side) in accordance with a demanded power (capacity) at an user side (indoor heat exchanger) in order to prevent overshoot or hunting of room temperature in cooling operation. A method of converting the frequency of alternating current by using an inverter to linearly control the driving rotational number of the compressor has been generally used to control the power (capacity) of the compressor. According to this method, the power of the compressor is freely variable from 0 to the rating point of the compressor, and thus the air conditioner can be substantially perfectly controlled. However, the inverter itself has various problems that energy loss due to frequency conversion is unavoidable, that it emits undesired electromagnetic wave to the environment and that a large-scale inverter increases the cost of the apparatus.
Therefore, Japanese Laid-open Patent Application No. Hei-8-247560 has proposed a power-variable type constant-speed compressor in which the power control is performed by a power save mechanism and a refrigerant return circuit while using a constant-speed compressor containing a compressor mechanism which is drive at a constant speed. According to the power save mechanism, a valve device is provided to the cylinder side wall or the like of a compression mechanism, and the compressing action is not performed at a first half of a compression process by opening the valve device, for example. Further, according to the refrigerant return circuit, a bypass circuit is provided between a refrigerant circuit at a discharge side of the compressor and a refrigerant circuit at a suck-in side of the compressor, and a part of refrigerant after the compression is circulated into the suck-in side refrigerant circuit.
When a power-variable type constant-speed compressor and a normal constant-speed compressor are combined, the multistage power control can be performed by individually driving or stopping both the compressors or using the power save mechanism and/or the refrigerant return circuit. For example, assuming that the rating power of the power-variable type constant-speed compressor is set to 4 horsepower, the rating power of the constant-speed compressor is set to 6 horsepower, the power reduction of the capacity-variable type constant-speed compressor by the power save mechanism is set to 2 horsepower and the power reduction of the refrigerant return circuit is equal to 1 horsepower, the power can be controlled every horsepower in the range of 1 to 10 horsepower (i.e., at 10 stages).
When the refrigerant return circuit as described above is opened, a part of refrigerant after the compression is circulated into the suck-in side refrigerant circuit, and thus the compressor carries out a vain compressor work. For example, when the driving is performed with 9 horsepower, the compression work of 1 horsepower is discarded by the refrigerant return circuit, however, the energy consumption is substantially equal to that when the driving is performed with 10 horsepower. Accordingly, an energy loss which is equal to or more than that of the inverter occurs, and this is a factor of making it difficult to use the capacity-variable type constant-speed compressor. Besides, it may be considered that no refrigerant circuit is provided and the power control is performed only the power same mechanism. In this case, in the construction of the above compressor, the power control is performed every 2 horsepower (i.e., at five stages). Therefore, in the air conditioner, overshoot or hunting of room temperature occurs if the power demand at the user side is small (for example, about 1 to 3 horsepower), and thus comfortableness of a user in a room to be air-conditioned may be lost.